Potassium titanyl phosphate (KTP) is an excellent nonlinear optical (NLO) material with numerous nonlinear optical applications. It is widely used for the second harmonic generation (SHG) of Nd: YAG/YLF lasers, sum frequency generation (SFG) of YAG, dye, and diode lasers, and optical parametric oscillation (OPO) in nanosecond to femtosecond regimes. More recently, applications of KTP for SHG and SFG of short wavelength diode lasers in waveguided, intracavity, and external resonator geometries have been rapidly developing. Isomorphs of KTP, such as potassium titanyl arsenate (KTA), are known to have similar nonlinear optical properties. Their application in nonlinear optics is also being developed.
Phase-matching is one of the key considerations for all applications involving frequency conversion. Phase-matching describes a propagation condition in which all electromagnetic waves travel through the NLO material with matching velocities such that the wave vectors total zero. (i.e., for three wave interaction, k.sub.1 +k.sub.2 -k.sub.3 =0; where k.sub.m =i.omega..sub.m n.sub.m /c, and where k.sub.m, .omega..sub.m, n.sub.m, and i denote the wavevector, frequency, index of refraction, and unit vector of propagation direction for each wave (m=1, 2, or 3) respectively, and c is the speed of light.) For most applications, phase-matching is simply achieved through the use of the natural birefringence in the NLO material. Depending on the polarization states of these waves, birefringence phase-matching is classified as type I and type II. Although KTP is type I phase-matchable for SHG throughout its transparency range, the type I effective nonlinearity is small and therefore not very useful (see F. C. Zumsteg, et al., J. Appl. Phy., 47, p. 4980 (1976)). For type II phase-matched SHG using KTP, where relatively large effective nonlinearity is accessed, the fundamental wavelength is limited to about 994.3 nm and longer, preventing efficient SHG into the technologically important blue region of the visible spectrum. Even for the phase-matchable wavelengths, the phase matching condition is generally very critical, requiring precise control of propagation direction in the material, of the wavelength, and of the material temperature. For KTP, phase-matched SHG of the Nd:YAG laser at 1.064 .mu.m has relatively tolerant conditions; with an angular bandwidth of about 15 mrad-cm, a wavelength bandwidth of about 0.56 nm-cm, and a temperature bandwidth of 25.degree. C.-cm (see J. D. Bierlein, et al., JOSA B, 6, p. 622 (1989)). In addition, double refraction can lead to spatial separation of the fundamental and 2nd harmonic waves (walk-off), limiting the interaction length and significantly reducing the performances in intracavity and resonator SHG applications.
For wavelengths that are phase-matched when propagating along a principle axis (generally known in the art as non-critical or 90.degree. phase-matching) the angular bandwidth restriction is significantly relaxed. For SHG using KTP, a wavelength of, 994.3 nm is noncritically phase-matched (NCPM) along the y axis, affording a broad angular bandwidth of 173 mrad-cm.sup.1/2, a wavelength bandwidth of 0.7 nm-cm, a temperature band width of 175.degree. C.-cm, and zero angular walk-off (see Risk et al., Appl. Phys. Lett., 55, p. 1179 (1989)). Along the x axis, where the effective nonlinearity is twice that along the y axis, the NCPM wavelength is at 1.08 .mu.m, with angular bandwidth that is about four times that of the critically phase-matched case at 1.064 .mu.m as well as zero walk-off (see Garmash et al., Tech. Phys. Lett., 12, p. 505, (1986)). Also, with NCPM the bandwidths decrease as the square-root of crystal length, instead of the linear dependence in critical phase-matching. Clearly, NCPM is highly desirable, especially for applications with long crystals and with multimode lasers. However, for pure KTP, given its natural birefringence and dispersion, NCPM is limited to two discrete wavelengths where there are no convenient laser sources. Similar considerations can be made for KTA and the other KTP isomorphs. There is a need for materials with increased birefringence to provide type II phase matching for other optical wave inputs.